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Best Practices for Image Capture1
(Prepared for California Digital Library, 7/99)
This document outlines a set of "best practices" for libraries, archives, and museums
who wish to create digital representations of parts of their collections. The
recommendations here focus on the initial stages of capturing images and metadata, and
do not cover other important issues such as systems architecture, retrieval,
interoperability, and longevity. Our recommendations are directed towards institutions
that want a large collection of their digital surrogates to persist for some prolonged period
of time. Institutions with very small collections or those who anticipate relatively short
life-spans for their digital surrogates may find some of the recommendations too
burdensome.
These recommendations really focus on reformatting existing works (such as
handwritten manuscripts, typescript works on paper, bound volumes, slides, or
photographs) into bitmapped digital formats. Because collections differ widely in their
types of material, audience, and institutional purpose, specific practices may vary from
institution to institution as well as for different collections within a single institution.
Therefore, the sets of recommendations we make here attempt to be broad enough to
apply to most cases, and try to synthesize the differing recommendations previously made
for specific target collections/audiences (for references to these previous documents, see
Bibliography).
Because image capture capabilities are changing so rapidly, we chose to divide the
"best practices" discussion into two parts: general recommendations that should apply to
many different types of objects over a prolonged period of time, and specific minimum
recommendations that take into consideration technical capabilities and limitations faced
by a hypothetical large academic library in 1999. The first part will cover the practices
that we think are fairly universal, and will be usable for many years to come. This
includes the notion of masters and derivatives, and some discussion about image quality
and file formats. The Summary of General Recommendations, found near the end of this
document, provides a list of these suggested best practices.
Since the issues surrounding image quality and file formats are complex, vary from
collection to collection, and are in flux due to rapid technological developments and
emerging standards, we have also summarized at the end of this document some more
1
This document was originally drafted by Howard Besser and edited and enhanced by the UC Berkeley
Image Capture and Presentation Task Force (1998). The first published edition of this document appeared
within the Digital Library Federation's Making of America II White Paper
(http://sunsite.berkeley.edu/moa2). That version was then enhanced by a committee composed of UC
Berkeley's Howard Besser, Dan Johnston, Jan Eklund, and Roy Tennant, then revised to its present form by
the California Digital Library's Architecture Committee.
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specific recommendations that provide a list of minimally acceptable levels rather than a
precise set of guidelines (see: Specific Minimum Recommendations for a sample
Project).
Digital Masters
Digital master files are created as the direct result of image capture. The digital master
should represent as accurately as possible the visual information in the original object.2
The primary functions of digital master files are to serve as a long-term master image and
as a source for derivative files. Depending on the collection, a digital master file may
serve as a surrogate for the original, may completely replace originals or be used as
security against possible loss of originals due to disaster, theft and/or deterioration.
Derivative files are created from digital master images for editing or enhancement,
conversion of the master to different formats, and presentation and transmission over
networks. Typically, one would capture the master file at a very high level of image
quality, then would use image processing techniques (such as compression and resolution
reduction) to create the derivative images (including thumbnails) which would be
delivered to users.
Long term preservation of digital master files requires a strategy of identification,
storage, and migration to new media, as well as policies about image use and access to
them. The specifications for derivative files used for image presentation may change over
time; digital masters can serve an archival purpose, and can be processed by different
presentation methods to create necessary derivative files without the expense of digitizing
the original object again. Because the process of image capture is so labor intensive, the
goal should be to create a master that has a useful life of at least fifty years. Therefore,
collection managers should anticipate a wide variety of future uses, and capture at a
quality high enough to satisfy these uses. In general, decisions about image capture
should err towards the highest quality.
Some collections will need to do image-processing on files for purposes such as
removing blemishes on an image, restoring faded colors from film emulsion, or
annotating an image. For these purposes we strongly recommend that a master be saved
before any image processing is done, and that the “beautified” image be used as a
submaster to generate further derivatives. In the future, as we learn more about the side
effects of image processing, and as new functions for color restoration are developed, the
original master would still be available.
2
Note: this is similarity in terms of technical fidelity using objective technological measurements; this is not
accuracy as determined by comparing the object scanned to an image on a display device. This type of
accuracy is obtained by the manipulation of settings before scanning rather than by image processing after
scanning. See further discussion of color pallette and monitors in the “Image Quality” section later in this
document.
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Capturing the Image
Appropriate scanning procedures are dictated by the nature of the material and the
product one wishes to create. There is no single set of image quality parameters that
should be applied to all documents that will be scanned. Decisions as to image quality
typically take into consideration the research needs of users (and potential users), the
types of uses that might be made of that material, as well as the artifactual nature of the
material itself. The best situation is one where the source materials and project goals
dictate the image quality settings and the hardware and software one employs. Excellent
sources of information are available, including the experience of past and current library
and archival projects (see Bibliography section entitled “Scanning and Image Capture”).
The pure mechanics of scanning are discussed in Besser (Procedures and Practices for
Scanning), Besser and Trant (Introduction to Imaging) and Kenney’s Cornell manual
(Digital Imaging for Libraries and Archives). It is recommended that imaging projects
consult these sources to determine appropriate options for image capture. Decisions of
quality appropriate for any particular project should be based on best anticipation of use
of the digital resource.
Image Quality
Image quality for digital capture from originals is a measure of the completeness and
the accuracy of the capture of the visual information in the original. There is some
subjectivity involved in determining completeness and accuracy. Sometimes the
subjectivity relates to what is actually being captured (with a manuscript, are you only
trying to capture the writing, or is the watermark and paper grain important as well?). At
other times the subjectivity relates to how the informational content of what is captured
will be used. ( For example, should the digital representation of faded or stained
handwriting show legibility or reflect the illegibility of the source material? Should pink
slides be “restored” to their proper color? And if the digital image is made to look
“better” than the original, what conflicts does that cause when a user comes in to see the
original and it looks “worse” than the onscreen version? See Sidebar II for more
complete discussion of these problems.). Image quality should be judged in terms of the
goals of the project, and ultimately depends on an understanding of who are the users
(and potential users), and what kind of uses will they make of this material. In past
projects, some potential use has been inhibited because not enough quality (in terms of
resolution and/or bit-depth) was captured during the initial scanning.
Image quality depends on the project's planning choices and implementation. Project
designers need to consider what standard practices they will follow for input resolution
and bit depth, layout and cropping, image capture metric (including color management),
and the particular features of the capture device and its software. Benchmarking quality
(see Kenney’s Cornell Manual) for any given type of source material can help one select
appropriate image quality parameters that capture just the amount of information needed
from the source material for eventual use and display. By maximizing the image quality
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of the digital master files, managers can ensure the on-going value of their efforts, and
ease the process of derivative file production.
Quality is necessarily limited by the size of the digital image file, which places an
upper limit on the amount of information that is saved. The size of a digital image file
depends on the size of the original and the resolution of capture (number of pixels per
inch in both height and width that are sampled from the original to create the digital
image), the number of channels (typically 3: Red, Green, and Blue: "RGB"), and the bit
depth (the number of data bits used to store the image data for one pixel).
Measuring the accuracy of visual information in digital form implies the existence of
a capture metric (i.e., the rules that give meaning to the numerical data in the digital
image file). For example, the visual meaning of the pixel data Red=246, Green=238,
Blue=80 will be a shade of yellow, which can be defined in terms of visual
measurements. Most capture devices capture in RGB using software based on the video
standards defined in international agreements. A thorough technical introduction to these
topics can be found in Poynton's Color FAQ:
<http://www.inforamp.net/~poynton/ColorFAQ.html>. We strongly urge that imaging projects
adopt standard target values for color metrics as Poynton discusses, so that the project
image files are captured uniformly.
A reasonably well-calibrated grayscale target should be used for measuring and
adjusting the capture metric of a scanner or digital camera.3 For capturing reflective
copy, we recommend that a standard target consisting of grayscale, centimeter scale
(useful for users to make sure that the image is captured or displayed at the right size),
and standard color patches be included along one edge of every image captured, to
provide an internal reference within the image for linear scale and capture metric
information. Kodak makes a set consisting of grayscale (with approximate densities),
color patches, and linear scale which is available in two sizes: 8 inches long (Q-13, CAT
152 7654) and 14 inches long (Q-14, CAT 152 7662)
Bit depth is an indication of an image's tonal qualities. Bit depth is the number of bits
of color data which are stored for each pixel; the greater the bit depth, the greater the
number of gray scale or color tones that can be represented and the larger the file size.
The most common bit depths are:



3
Bitonal or binary, 1 bit per pixel; a pixel is either black or white
8 bit gray scale,; 8 bits per pixel; a pixel can be one of 256 shades of gray
8 bit color, 8 bits per pixel ("indexed color"); a pixel is one of 256 colors
Targets for source material that themselves are intermediates (eg. slides, microfilm, photocopies) may be
more confusing than helpful. Targets are not reasonable for slides because of the small size. Targets for
other intermediates can be misleading, as future users employing them to adjust their viewing environment
to the image may be confused as to whether they are adjusting to the proper settings for viewing the
intermediate or to viewing the original.
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24 bit color (RGB), 24 bits per pixel; each 8-bit color channel can have 256 levels,
for a total of 16 million different color combinations
While it is desirable to be able to capture images at bit depths greater than 24 (which
only allows 256 levels for each color channel), standard formats for storing and
exchanging higher bit-depth files have not yet evolved, so that we expect that (at least for
the next few years) the majority of digital master files will be 24-bit. Project planners
considering bitonal capture should run some samples from their original materials to
verify that the information captured is satisfactory; frequently grayscale capture is
desirable even for bitonal originals. 8-bit color is seldom suitable for digital masters.
Useful image quality guidelines for different types of source materials are listed in
Puglia & Rosinkski’s NARA Guidelines and in Kenney’s Cornell Manual (see
bibliography).
Formats
Digital masters should capture information using color rather than grayscale
approaches where there is any color information in the original documents. Digital
masters should never use lossy compression schemes and should be stored in
internationally recognized formats. TIFF is a widely used format, but there are many
variations of the TIFF format, and consistency in use of the files by a variety of
applications (viewers, printers etc.) is a necessary consideration. In the future, we hope
that international standardization efforts (such as ISO attempts to define TIFF-IT and
SPIFF) will lead vendors to support standards-compliant forms of image storage formats.
Proprietary file formats (such as Kodak’s Photo CD or the LZW compression scheme)
should be avoided for any long-term project. Most projects currently use uncompressed
TIFF 6 images.
While it is our recommendation that no file compression be used at all for digital
master files, we recognize that there may be legitimate reasons for considering it.
Limited storage resources may force the issue by requiring the reduced file sizes that file
compression affords. Those who choose to go this route should be careful to take into
consideration digital longevity issues. As a general rule, lossless compression schemes
should be preferred over lossy compression schemes. Lossless compression makes files
smaller, and when they are decompressed they are exactly the same as before they were
compressed. Lossy compression actually combines and throws away data (usually data
that cannot be readily detected by the human eye), so decompressed lossy images are
different than the original image, even though those differences may be difficult for our
eyes to see. Typically, lossy compression yields far greater compression ratios than
lossless. But unlike lossy compression, lossless compression will not eliminate data we
may later find useful. Lossy compression is unwise, as we do not yet know how today’s
lossy compression schemes (optimized for human eyes viewing a CRT screen) may affect
future uses of digital images (such as computer-based analysis systems or display on
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future display devices). But even lossless compression adds a level of complexity to
decoding the file many years hence. And many vendor products that claim to be lossless
(primarily those that claim “lossless JPEG”) are actually lossy.
Image Metadata
Metadata or data describing digital images must be associated with each image
created, and most of this should be noted at the point of image capture. Image metadata
is needed to record information about the scanning process itself, about the storage files
that are created, and about the various pieces that might compose a single object.
The number of metadata fields may at first seem daunting. However, high
proportions of these fields are likely to be the same for all the images scanned during a
particular scanning session. For example, metadata about the scanning device, light
source, date, etc. is likely to be the same for an entire session. And some metadata, about
the different parts of a single object (such as the scan of each page of a book), will be the
same for that entire object. This kind of repeating metadata will not require keyboarding
each individual metadata field for each digital image; instead, these can be handled either
through inheritance or by batch-loading of various metadata fields.
Administrative metadata includes a set of fields noting the creation of a digital master
image, identifying the digital image and what is needed to view or use it, linking its parts
or instantiations to one another, and ownership and reproduction information. Structural
metadata includes fields that help one reassemble the parts of an object and navigate
through it. Descriptive metadata provides information about the original scanned object
itself, to support discovery and retrieval of the material from within a larger archive.
Derivative Images
Since the purpose of the digital master file is to capture as much information as is
practical, smaller derivative versions will almost always be needed for delivery to the end
user via computer networks. In addition to speeding up the transfer process, another
purpose of derivatives may be to digitally "enhance" the image in one form or another
(see discussion below of artifact vs. content) to achieve a particular goal. Such
enhancements should be performed on a submaster rather than on the digital master file
(which should reflect only what the particular digitization process has captured).
Derivative versions are typically not files that will be preserved, as the digital master file
is for that purpose.
Derivative images for web-based delivery might be pre-computed in batch mode from
masters early on in a project, or could be generated on demand from web-resident
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submasters on-the-fly as part of a progressive decompression function or through an
application such as MrSID (Multi-resolution Seamless Image Database).
Sizes
Typical derivative versions include a small preview or "thumbnail" version (usually
no more than 200 pixels for the longest dimension) and a larger version that mostly fills
the screen of a computer monitor (640 pixels by 480 pixels fills a monitor set at a
standard PC VGA resolution). Depending on the need for users to detect detail in an
image, a higher resolution version may be required as well. The full set and sizes of
derivative images required will depend upon a variety of factors, including the nature of
the material, the likely uses of that material, and delivery system requirements (such as
user interface). Derivative files should be created using software to reduce the resolution
of the master image, not by adjusting the physical dimensions (width and height). After
reducing the resolution, it may be necessary to sharpen the derivative image to produce an
acceptable viewing image (e.g., by using "unsharp mask" in Adobe Photoshop). It is
perfectly acceptable to use image processing on derivative images, but this should never
be done to masters.
Derivative images will frequently be compressed using lossy compression.
Compression algorithms are usually optimized for a particular type of image (e.g. JPEG
achieves high compression ratios for pictorial images, but cannot compress images of text
very much without introducing compression artifacts), and one should be careful not to
use the wrong type of compression scheme for a particular image. Compression
algorithms such as JPEG involve a spectrum of options (ranging from high compression
ratios that involve visible loss to low compression ratios that involve little visible loss).
Each software implementation of these options label them differently (on scales of 1-3,
scales of 1-10, options high/medium/low, etc.), and there is currently no objective and
interoperable way to declare which of a range of options one has chosen.
Artifactual vs. Enhanced
Many historical images are faded, yellowed, or otherwise decayed or distorted. Image
enhancement techniques can in some cases result in a much improved image for viewing
the content of the image rather than the decayed condition of the artifactual print or
transparency. In situations where such an enhanced viewing version is desired, it should
in most cases be offered in addition to a version that more closely depicts the condition of
the artifact. By having both images available, users will understand the condition of the
original while having a more useable version for online viewing.
Production of artifactual derivatives can often be automated by using software that
can perform a series of standard operations. Production of enhanced versions, however,
will most likely not be able to be automated, due to the inability of any one standard
transformation procedure to apply equally to all images in a particular project. If
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automated procedures for image enhancement are not effective, the costs of creating these
images individually will need to be considered in the overall project cost.
Color Management
The objective of color management is to control the capture and reproduction of color
in such a way that an original print can be scanned, displayed on a computer monitor, and
printed, with the least possible change of appearance from the original to the monitor to
the printed version. This objective is made difficult by the limits of color reproduction:
input devices such as scanners cannot "see" all the colors of human vision, and output
devices such as computer monitors and printers have even more limited ranges of colors
they can reproduce. Further, there is substantial variability in the color reproduction of
different capture, display and output devices, even among identical models from the same
manufacturer. Most commercial color management systems are based on the ICC
(International Color Consortium) data interchange standard, and are often integrated with
image processing software used in the publishing industry. They work by systematically
measuring the color properties of digital input devices and of digital output devices, and
then applying compensating corrections to the digital file to optimize the output
appearance. Although color management systems are widely used in the publishing
industry, there is no consensus yet on standards for how (or whether) color management
techniques should be applied to digital master files. Though projects may experiment
with color management systems for derivative files, until a clear standard emerges it is
not recommended that digital master files be routinely processed by color management
software.4
Strategies for Digital Preservation
How to keep digital files alive and accessible over time is a key issue facing our field,
and as yet little consensus has emerged (Waters et. al.). The two general approaches are
Emulation and Migration. Emulation assumes that we will keep files in their current
encoding format, and that software will eventually be written for future computers and
operating systems that will emulate current viewing and manipulation environments.
Migration requires that files be periodically copied from one encoding format to a newer
format (such as from WordStar to Microsoft Word 3 to Word 6 to Word 8, ...). Both
approaches assume that files will be periodically transferred from one physical medium to
another (to avoid deterioration of the tape, disk, or other storage media). Appropriate
metadata is critical to both approaches. Metadata is necessary to: know what file formats
the file requires, and to make sure that all related files are emulated or migrated.
Metadata will also be crucial for future environments which may include features such as
automatic color control, and measurement tools to compare image sizes.
4
Some image processing tools (such as Photoshop 5) default to implementing color management. Users
need to beware, and may need to turn off such functions.
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Summary of General Recommendations

Plan image capture projects with a goal of long-term access to the digital materials
created

Think about users (and potential users), uses, and type of material/collection

Scan at the highest quality you can possibly justify based on potential
users/uses/material. Err on the side of quality.

Do not let today’s delivery limitations influence your scanning file sizes; understand
the difference between digital masters and derivative files used for delivery

Many documents which appear to be bitonal actually are better represented with
grayscale scans; some documents that appear to be grayscale are better represented in
color

Include grayscale target, standard color patches, and ruler in the scan

Use objective measurements to determine scanner settings (do NOT attempt to make
the image good on your particular monitor or use image processing to color correct)

Don’t use compression for digital masters

Store in a common (standardized) file format

Capture as much metadata as is reasonably possible (including metadata about the
scanning process itself)
Specific Minimum Recommendations for a hypothetical large
academic library in 1999
[[[insert table here]]]
Some originals which may appear to be candidates for grayscale scanning may in fact be better represented
by color capture (usually when the artifactual nature of a historical item needs to be retained); in those
cases, use the color master recommendations.
2
Suggestions that we need to make sure have been integrated:
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Working through the Imaging Decisions: The Honeyman
Project (SideBar I)
In September of 1998 the Bancroft Library and the Library Photo Service completed a
one-year project with an important digital imaging component: the Robert B. Honeyman,
jr. Collection Digital Archive Project. The resulting work is now available as part of the
California Heritage website at http://sunsite.berkeley.edu/CalHeritage
With hindsight, we can look back on the image capture decisions, and consider how
they influenced the project results.
Project goals:
The main goals of the project were to provide public access to images of each item in
the collection for purposes of study and teaching, and to provide improved intellectual
control of the collection by compiling an on-line catalog with links to the images.
Project scope:
The Honeyman collection in the Bancroft Library includes about 2300 items related to
California and Western US history, including oil paintings, lithographs, sketches, maps,
lettersheets, items in scrapbooks, and more.
Project Image requirements
The Honeyman project calls for web-viewing files in keeping with the design of the
California Heritage website, which offers each image at three resolutions (derived from a
master): a small thumbnail version, for browsing; a larger "medium resolution" version,
sized to approximately fit on a typical computer screen; and a "high resolution" version
for viewing image details, at twice the resolution of the "medium" version. Other
considerations (discussed below) led to the decision to capture the digital masters at a
higher level of resolution than was required to meet this minimal requirement.
Additional Imaging goals
Given that the added costs of improved image capture quality (over and above the
minimum necessary for the project) are often small compared with the costs of repeating
the project for more quality sometime in the future, some additional likely uses for the
digital captures from the project were considered. In the future it should be possible to
offer higher resolution versions of the images from the master for more detailed study via
the web. Also, these images can provide a permanent record of each item in the collection
and its condition at the time of capture. Finally, because images from the Honeyman
collection are frequently published, the archived digital images may be supplied to
publishers instead of photographs made conventionally.
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Imaging Equipment Available
In addition to traditional (film) cameras, the Library Photographic Service has a
digital camera: a PhaseOne Powerphase scanning back which fits on a Hasselblad
camera. The Powerphase is used on a copy stand and controlled from a PC (personal
computer, either Powermac or Wintel); it is capable of capturing an image measuring
7000 pixels square. Also, an Epson 836 flatbed scanner became available during the
project; it can scan originals as large as 12x17 inches at 800 ppi (pixels per inch).
Capture Specifications for Digital Masters
File Format
Digital masters are captured in 24 bit RGB color and stored in uncompressed TIFF
format, as managed by the PhaseOne software. In the case of the Epson scanner, the
scanner software runs as a plug-in to Adobe Photoshop, so the file is created by
Photoshop 5. This file format is widely readable.
Capture Resolution and Master File Size
Because of the wide range of sizes and types of originals represented in the
Honeyman collection, no single value for capture resolution could be set. Instead, a
number of discrete resolution values were used, and originals were sorted prior to capture
into groups suitable for each resolution, based on the capacity of the Powerphase camera.
(The camera's capture resolution is adjusted by moving the camera on the copystand
nearer or farther from the subject; the nearer the camera, the higher the capture resolution
and the smaller the area of coverage). The highest resolution used was 600 ppi; at this
resolution the camera will capture an area about 11.5 inches square, a comfortable fit for
an 8-1/2 x 11 inch original. Several arguments were used to establish 600 ppi as the
preferred resolution level for capture: 600 ppi is sufficient to capture extremely small text
legibly; it is sufficient for high-quality publication at double life-size; and the resulting
TIFF file sizes (up to 143 MB) are small enough to be processed and stored on suitablyequipped PC's. This is the sorting table:
Resolution Coverage
(ppi)
(inches)
600
11.5
450
15.5
300
23
200
35
150
46
(for example, an 11x17 inch original would be copied at 300 ppi because it fits in a 23x23
inch capture area, but can't be completely covered in the 15.5x15.5 inch area covered at
450dpi)
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As a result of applying the sorting table, most of the capture files are in the range of
5000 to 6000 pixels in the long dimension, and their file sizes tend to be between 60 and
100 MB.
The file processing workflow was developed and tested to handle file sizes up to
about 140 MB.
Included Targets
A one-piece target is imaged at the edge of each capture. It combines the grayscale
target and the color patches from a Kodak Q-13 Color Separation Guide and Grayscale
with a centimeter scale, all in a very compact layout created using a hobby knife and twosided adhesive tape. The information from the target is intended to provide information
about the tonality and scale of the image to scholars and technicians. The crucial "A,"
"M," and "B" steps of the grayscale are marked with small dots to make them easy to
identify for making tonal measurements (see ‘Tonal Metric’ section below) during
capture set-up and file processing. Several different-sized versions of the combined target
are suited to the range of sizes of the originals.
Tonal Metric
The RGB data in the image files is captured in the native colorspace of the capture
device (camera or scanner); that is, no color management step such as applying a
Colorsync profile is used on the digital masters prior to saving. Before capture occurs, the
camera (or scanner) operator uses the controls in the scanning software to adjust the color
balance, brightness, and contrast of the scan so that the grayscale target in the image has
the expected RGB values. These values are as follows: for the white "A" patch, R, G, and
B values all at or near 239; for the middle-gray "M" patch, RGB = 98; for the near-black
"B" patch, RGB = 31. These expected RGB values are appropriate for a 24 bit RGB
image with gamma 1.8.
Cropping and Background
Originals are depicted completely, including blank margins, against a light gray
background paper (Savage "Slate Gray"), so that the digital image documents the physical
artifact, as well as reproducing the imagery that the artifact portrays. A narrow gap
between the grayscale target and the original allows for cropping the grayscale out of the
composition if desired for some new purpose.
Adapting to Different Formats of Originals
Some aspects of image capture were determined or suggested by the formats of the
originals:
 Subcollections suitable for flatbed scanning: The lettersheets, clipper ship cards, and
sheetmusic covers are all small enough to fit on the 12x17 inch platen of the Epson
scanner, so that these groups were routed to the flatbed. By operating both the scanner
and the camera in tandem, the production rate of image capture was increased.
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Miscellaneous flat art: this group, mostly stored in folders, represents the bulk of the
Honeyman Collection, including watercolors, lithographs, maps, etc. Many items are
mounted on larger backing sheets, and many are in mounts with window mats,
making flatbed scanning difficult, even if size and fragility are not an issue. These
items were sorted by capture resolution and routed to the Powerphase.
Bound originals: scrap books and sketch books were captured with the digital camera.
In cases where more than one item per page is cataloged, separate digital captures
were made for each item; otherwise, the entire page is recorded.
Framed works: the 136 framed items in the Collection were captured using a film
intermediate. They were photographed at the Bancroft Library using 4x5 Ektachrome
film, and the film was scanned on the Epson scanner. This was done to minimize the
handling of these awkward and vulnerable objects in two ways: the originals would
not have to be shuttled between the Bancroft Library and the Photographic Service, at
opposite ends and levels of the Library complex, for the project; and the 4x5 film
intermediates can be made available to publishers as need arises without further
handling of the originals (no system is in place as yet to supply digital files to
publishers).
Storing the Digital Masters
The digital capture files were recorded on external hard drives at the capture stations;
the hard drives were than moved to other PC's equipped with CD-Recordable drives,
where the digital master files were transferred to CD-R disks.
Specifications for the Viewing Files
The viewing files for the Honeyman web site are designed for convenient
transmission over the Internet and satisfactory viewing using typical PC hardware and
web browser software:
 A thumbnail GIF, maximum dimension 220 pixels
 A medium format JPEG-compressed (JFIF) version, maximum dimension 750 pixels,
transmitted filesize around 50 KB.
 A higher-resolution JPEG version, maximum dimension 1500 pixels, transmitted
filesize roughly 200 KB.
 Gamma adjustment: the viewing files are prepared for a viewing environment of
gamma 2.2
Making the Viewing Files
The viewing files were made from the CD-R files in batches, on yet another PC
running Adobe Photoshop 5 or Debabelizer 3. The master files are opened, gammaadjusted, downsampled to size, sharpened with unsharp mask, and saved in GIF and
JPEG formats. The derivatives from digital camera files are also color-managed with a
Colorsynch profile created using Agfa Fototune Scan.
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Thoughts on Digital Capture from Film-based Formats
(SideBar II)
Digital masters should record the visual appearance of the original artifact in a welldefined way, so that later users can knowledgeably interpret and manipulate the data, and
so that capture technicians can follow consistent procedures for capture and quality
control. For most kinds of documents and flat artwork, this can be accomplished by
defining the desired digital values for standard targets (primarily a grayscale) included in
the imaging. However, originals in other formats such as slides and negatives present
different challenges.
For direct digital capture of negative collections, the visual appearance of the negative
would ordinarily be of little interest; it is the visual appearance of the positive image
created from the negative that is usually central. Methods and materials for making
negatives have changed repeatedly over the years, and importantly, so have the materials
and practices used in making prints from them. Planners may choose to express their
capture plans in terms of the properties of the negative (e.g. transmission density), or try
to describe a positive master format to be captured directly from the negative. Either
course has pitfalls. While the first may seem like the safer approach, fully describing a
negative will probably require more data than 8 bits per channel can carry, because
negatives routinely record a much wider range of tones than media such as prints. It
would be advisable for the planners to fully consider each step of the image
transformation, from the densities in the negative, to raw scanned data, to digital master,
to derivative products, making sure that enough of the right information is available in the
master to satisfy the needs of derivative production. Also, as an aside, it's useful to
remember that even black and white negatives often contain color information such as
stained regions; scanning such a negative in color often reveals that one color channel
emphasizes the effect of the stain, while another color channel may hide it.
Planners contemplating a project to scan color slides, some of which are faded, may
find they need to make choices about whether to try to correct for the fading at the time of
scanning, or to store a "faded", realistic digital master and then possibly produce a
"restored" derivative to satisfy a need for an unfaded product. The second method would
be favored because it provides for both a realistic and a "restored" version, and offers the
possibility that different, better "restored" versions can be created in the future, regardless
of any ongoing changes in the condition of the slide. However, in some cases the scanner
and its software may be better able to correct the faded color channels at the time of
scanning by tailoring the information-gathering to the levels of each dye remaining. The
purposes and priorities of the project can help make the necessary choices: if a purpose of
the digital project is to give lecturers a tool for selecting lecture slides for projection, or to
record the condition of the slides, then the project will require a product that faithfully
represents the faded condition of the slide. Experimental trials can reveal whether the
"restoration" is better if performed at the time of scanning for a particular scanner and
Best Practices
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level of fading; in some cases multiple scannings and multiple masters may be the
solution.
Another question comes up when a collection of slides that depict documents or
works of art is to be scanned: should the digital master strive to represent the slide, or the
original the slide depicts? On one hand, if the slide itself is considered an original work,
the digital master should probably represent the slide as accurately as possible. On the
other hand, many digital image capture projects involve a film intermediate: the
document to be captured is first photographed on film, then the film is scanned to create
the master file. In this case, the film may be seen as a vehicle for recording the tonality of
the original document, and the scanning of the film may be adjusted so as to correct for
the color and tonal errors introduced in the filming. The inclusion of a gray card or
grayscale, photographed together with the work of art, can make objective corrections
possible.
Digital capture of faded documents presents many of the same challenges as scanning
faded slides: is the objective to show the appearance of the document in its present
condition, to make it maximally legible, or perhaps to depict it as we imagine it appeared
as new? Ordinarily the digital master would be made to depict the document as it exists,
and the master would then be processed to create the legible or "reconstructed"
derivatives. However, in some situations faded information may be better captured using
extreme, non-realistic means such as narrow-band light filters, or invisible wavelengths
such as infrared. In these cases, multiple captures and multiple digital masters may be
appropriate.
Microfilm is a photographic medium designed not for natural, realistic tonal capture
but for optimal legibility. A project to capture digital images from microfilm taken of
manuscript originals might naturally choose to emphasize legibility over tonal accuracy in
its masters and derivatives since the microfilm intermediate is already inclined that way;
this would be an important consideration in choosing whether microfilm is an appropriate
source for scanning for a particular purpose.
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Bibliography
[[need some standardization of citation formats]]
Metadata
[Making of America II White Paper], Part III, Structural and Administrative Metadata
http://sunsite.berkeley.edu/MOA2
[Cornell University Library] METADATA WORKING GROUP REPORT to Senior
[Library] Management, JULY 1996
http://www.library.cornell.edu/DLWG/MWGReporA.htmand the related work “Distillation of
[Cornell UL] Working Group Recommendations” November, 1996
http://www.library.cornell.edu/DLWG/DLMtg.html
“Information Warehousing: A Strategic Approach to Data Warehouse Development” by
Alan Perkins, Managing Principal of Visible Systems Corporation (White Paper
Series) http://www.esti.com/iw.htm
SGML as Metadata: Theory and Practice in the Digital Library. Session organized by
Richard Gartner (Bodleian Library, Oxford) http://users.ox.ac.uk/~drh97/Papers/Gartner.html
“A Framework for Extensible Metadata Registries” by Matthew Morgenstern of Xerox, a
visiting fellow of the Design Research Institute at Cornell
http://dri.cornell.edu/Public/morgenstern/registry.htm
Using the Library of Congress Repository model, developed and used in the National
Digital Library Program: The Structural Metadata Dictionary for LC Repository
Digital Objects http://lcweb.loc.gov:8081/ndlint/repository/structmeta.html which then leads to
further documentation of their Data Attributes
http://lcweb.loc.gov:8081/ndlint/repository/attribs.html with a list of the attributes
http://lcweb.loc.gov:8081/ndlint/repository/attlist.html and their definitions
http://lcweb.loc.gov:8081/ndlint/repository/attdefs.html The same site then gives examples of
using this model for a photo collection http://lcweb.loc.gov:8081/ndlint/repository/photosamp.html a collection of scanned page images
http://lcweb.loc.gov:8081/ndlint/repository/timag-samp.html and a collection of scanned page
images and SGML encoded, machine-readable text
http://lcweb.loc.gov:8081/ndlint/repository/sgml-samp.html
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Organization of Information for Digital Objects
The article “An Architecture for Information in Digital Libraries” by William Arms,
Christophe Blanchi and Edward Overly of the Corporation for National Research
Initiatives and published in D-Lib Magazine, February 1997
issue.http://www.dlib.org/dlib/february97/cnri/02arms1.html
Howard Besser. Digital Longevity (website) http://sunsite.Berkeley.edu/Longevity
Repository Access Protocol – Design Draft – Version 0.0 by Christophe Blanchi of CNRI
is found at http://titan.cnri.reston.va.us:8080/pilot/locdesign.html and begins “ This
document describes the repository prototype for the Library of Congress. This design
is based on version 1.2 of the Repository Access Protocol (RAP) and the Structural
Metadata Version 1.1 from the Library of Congress.”
“The Warwick Framework: A Container Architecture for Aggregating Sets of Metadata”
by Carl Lagoze, Digital Library Research Group, Computer Science Department,
Cornell University; Clifford A. Lynch, Office of the President, University of
California, and Ron Daniel Jr., Advanced Computing Lab, Los Alamos National
Laboratory (July, 1996) http://cstr.cs.cornell.edu/Dienst/Repository/2.0/Body/ncstrl.cornell/TR96-1593/html
Waters, Don, et. al. Preserving Digital Information: Report of the Task Force on the
Archiving of Digital Information, Washington DC: Commission on Preservation and
Access and Research Libraries Group, 1996 http://www.rlg.org/ArchTF/
Scanning And Image Capture
Adobe Systems Incorporated, TIFF Revision 6:
ftp://ftp.adobe.com/pub/adobe/devrelations/devtechnotes/pdffiles/tiff6.pdf
This textbook-length standard defines the detailed structure of TIFF
(Tagged Interchange File Format) graphics files for all standard TIFF
formats (except TIFF/IT, see next entry). If you want to decipher the
header information in a TIFF file, this provides the technical
documentation. It is also interesting to see the variety of options (many
rarely implemented) available to programmers within the standard,
including features like JPEG compression, and metadata tags such as
Copyright notice.
Howard Besser and Jennifer Trant. Introduction to Imaging. Getty Art History
Information Project. http://www.gii.getty.edu/intro_imaging/
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Still the best overview of electronic imaging available for the beginner
and should be considered recommended reading for any level. Starts
with a basic description of what a digital image is, and continues with a
discussion of the basic elements that need to be considered before,
during, and after the capture process. Includes a detailed discussion of
the image capture process, compression schemes, uses, as well as access
to and documentation of the final product. Covers selection of scanning
equipment, image-database software, network delivery, and security
issues. Includes a top-notch glossary and links to many useful resources
on the WWW. Highly recommended. -JE
Howard Besser. Procedures & Practices for Scanning, Procedures and Processes for
Scanning. Canadian Heritage Information Network (CHIN),
http://sunsite.Berkeley.edu/Imaging/Databases/Scanning
Posed as a series of questions to ask about the task at hand, this
document outlines, in a very straightforward way, all the basic decisions
that must be made before, during, and after the scanning process.
Includes both abbreviated, and longer explanations of each point,
making it useful to a wide audience with varying levels of technical
sophistication.
Electronic Text Center at Alderman Library, University of Virginia. "Image Scanning: A
Basic Helpsheet, http://etext.lib.virginia.edu/helpsheets/scanimage.html
A very straightforward, basic, how-to document that outlines the image
scanning process at the Electronic Text Center at Alderman Library,
University of Virginia. Includes a good, concise discussion of image
types, resolution, and image file formats, as well as a brief discussion
about "Archival Imaging" and associated metadata. Also includes more
specific recommendations for using Adobe Photoshop and DeskScan
software with an HP Scanjet flatbed scanner.
Electronic Text Center at Alderman Library, University of Virginia. "Text Scanning: A
Basic Helpsheet” http://etext.lib.virginia.edu/helpsheets/scantext.html
A very concise description of the optical character recognition process
that converts scanned images into text at the Electronic Text Center at
Alderman Library, University of Virginia. Outlines the process in a
step-by-step fashion, assuming the use of the Etext Center equipment,
which consists of a pentium PC, an HP Scanjet flatbed scanner, and
OmniPage Pro version 8 OCR software.
Michael Ester. Digital Image Collections: Issues and Practice. Washington, D.C. ,
Commission on Preservation and Access (December, 1996).
This pithy report is riddled with useful insights about how and why
digital image collections are created. Ester is especially effective at
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pointing out the hidden complexities of image capture and project
planning, without ever getting too technical for a general audience.
Carl Fleischhauer. Digital Historical Collections: Types, Elements, and Construction.
National Digital Library Program, Library of Congress,
http://lcweb2.loc.gov/ammem/elements.html.
One of three articles that cover the Library of Congress digital
conversion activity as of August 1996. Discussion covers the types of
collections converted, the access aids established for online browsers,
types of digital reproductions made available (images, searchable text,
sound files, etc.), and supplementary programs known as "Special
Presentations" on the Library website. Describes the developmental
approach to assigning names to digital elements, and how those elements
are identified as items and aggregates. Brief descruption of how digital
reproductions are being used in preservation efforts.
Carl Fleischhauer. Digital Formats for Content Reproductions. National Digital Library
Program, Library of Congress. http://lcweb2.loc.gov/ammem/formats.html
A clear explanation of capturing digital representations of different types of
materials.
One of three articles that cover the Library of Congress digital
conversion activity as of August 1996. Discussion documents Library's
selection of digital formats for pictorial materials, textual materials,
maps, sound recordings, moving-image materials, and computer file
headers. Includes specific target values for tonal depth, file format,
compression, and spatial resolution for the image and text categories.
Good discussion of various mitigation measures that might be employed
to reduce or eliminate moire patterns that result from the scanning of
printed halftone illustrations and MrSID approach to map files. Sound
recording and moving-image files are documented with the caveat that
these will likely change in the near future as the technology evolves.
Image Quality Working Group of ArchivesCom, a joint Libraries/AcIS Committee.
Technical Recommendation for Digital Imaging Projects,
http://www.columbia.edu/acis/dl/imagespec.html
Contains a good Quick Guide, in table format, that recommends a specific
conversion method, capture resolution, archive file format, screen format, and
presentation format for 6 media types: 1) black and white text documents, 2)
illustrations, maps & manuscripts, 3) 3-D objects, 4) 35mm slides & negatives, 5)
medium to large format photos, negatives and transparencies or color microfiche,
and 6) black and white microfilm. Clear, brief explanations for when to use film
intermediaries, and more specific recommendations for bit-depth, capture
resolution, and file formats for archival storage and presentation. Includes
estimated file sizes for different file formats to facilitate the calculation of
physical storage requirements.
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International Color Consortium: http://color.org/
The ICC is a consortium of major companies involved with digital
imaging, formed to create industry-wide standards for digital color
management. Systems such as Apple's Colorsync, Agfa's Fototune, and
Microsoft's Integrated Color Management (II) which follow the
standards are said to be "ICC compliant" and may be able to exchange
information. The ICC web site offers a downloadable version of the
standards along with other technical papers.
International Organization for Standardization, Technical Committee 130 (n.d.).
ISO/FDIS 12639: Graphic technology – Prepress digital data exchange – Tag image
file format for image technology (TIFF/IT). Geneva: International Organization for
Standardization (ISO).
This standard defines an extension of the TIFF 6.0 standard specifically
intended to apply to graphics files used by prepress applications in the
printing industry. In addition to standardizing the formats used for
electronic dissemination of page layouts, etc., it calls for backwardcompatibility with common TIFF 6-compatible applications such as
Adobe Photoshop, to minimise proliferation of TIFF formats that can’t
be opened by many applications.
Anne R. Kenney. Digital Imaging for Libraries and Archives. Cornell University Library,
June 1996.
A very thorough and useful resource for digital imaging projects. The section on
hardware is a bit dated, but most of the publication is still very useful. The
explanations of digital imaging technology are useful, as is the advice for
anyone tackling a digitization project for the first time. Text is VERY dense but
is a good in-depth technical overview of the issues involved in the scanning
process. The formulas are useful (if complex) and are better when it comes to
text scanning than image scanning. Emphasis on benchmarking. Not for the
faint of heart.
Picture Elements, Inc. Guidelines for Electronic Preservation of Visual Materials
(revision 1.1, 2 March 1995). Report submitted to the Library of Congress,
Preservation Directorate.
Poynton's Color FAQ: http://www.inforamp.net/~poynton/ColorFAQ.html
This is one of several papers at this site by Charles Poynton introducing
the technical issues of color and tonal reproduction in the digital realm
for general audiences. Some of the explanations are accessible to
everyone, such as how to adjust the brightness and contrast on a PC
monitor; others include some math, such as how to transform from one
color space to another. Many useful references are cited. Included are
discussion of human visual response, primary colors, and the televisionbased standards underlying digital imaging.
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Steven Puglia and Barry Roginski. NARA Guidelines for Digitizing Archival Materials
for Electronic Access, College Park: National Archives and Records Administration,
January 1998. http://www.nara.gov/nara/vision/eap/digguide.pdf
An exhaustive and specific set of guidelines for digitizing textual, photographic,
maps/plans, and graphic materials. Includes specific guidelines about resolution
and images size for master files, access files, and thumbnail files. Also includes
specific guidelines for scanner/monitor calibration, and file header information
tags. Handy quick reference chart at
http://www.nara.gov/nara/vision/eap/digmatrx.pdf.
Reilly, James M and Franziska S. Frey, "Recommendations for the Evaluation of Digital
Images Produced from Photographic, Microphotographic, and Various Paper
Formats" Report to the Library of Congress, National Digital Library Project by
Image Permanence Institute. May, 1996 http://lcweb2.loc.gov/ammem/ipirpt.html
This report is intended to guide the Library of Congress in setting up
systematic ways of specifying and judging digital capture quality for
LC's digital projects. It includes some interesting discussion about digital
resolution, but discussion of tonality and color is brief.
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Specific Minimum Recommendations for a hypothetical large
academic library in 1999
nal material
/W text
ocument
22
resolution in
longest
dimension
6000 pixels
3000 pixels6
digital master
color depth
8 bit
grayscale
file format
TIFF with
lossless
compression
TIFF with
lossless
compression
strations,
Maps,
uscripts, etc.
6000 pixels
3000 pixels
24 bit color
mensional
ects to be
esented in
dimensions
m film, color
gative or
positive
6000 pixels
3000 pixels
24 bit color
TIFF with
lossless
compression
6000 pixels
3000 pixels
24 bit color
TIFF or PhotoCD
with lossless
compression
m film, B/W
gative or
positive
6000 pixels
3000 pixels
8 bit
grayscale
TIFF or PhotoCD
with lossless
compression
um to large
mat color
otograph,
egative,
parency or
icrofiche
ofilm, B/W
6000 pixels
3000 pixels
24 bit color
TIFF or PhotoCD
with lossless
compression
6000 pixels
3000 pixels
8 bit
grayscale
TIFF with
lossless
compression
5
resolution in
longest
dimension5
1200 pixels
600 pixels
screen display
color depth
file format
resolution in
longest
dimension
3000 pixels or
6000 pixels
print display
color depth
8 bit
grayscale
GIF or JFIF
300 pixels
800 pixels
1500 pixels
3000 pixels
300 pixels
800 pixels
1500 pixels
3000 pixels
300 pixels
800 pixels
1500 pixels
3000 pixels
300 pixels
800 pixels
1500 pixels
3000 pixels
300 pixels
800 pixels
1500 pixels
3000 pixels
24 bit color
JFIF, quality
level 50
3000 pixels
24 bit color
24 bit color
JFIF, quality
level 50
3000 pixels
24 bit color
JFIF, qua
level 50-1
24 bit color
JFIF, quality
level 50
3000 pixels
24 bit color
JFIF, qua
level 50-1
8 bit
grayscale
JFIF, quality
level 50
3000 pixels
8 bit
grayscale
JFIF, qua
level 50-1
24 bit color
JFIF, quality
level 50
6000 pixels
24 bit color
JFIF, qua
level 50-1
300 pixels
800 pixels
1500 pixels
3000 pixels
8 bit
grayscale
JFIF, quality
level 50
6000 pixels
3000 pixels
8 bit
grayscale
PDF or T
with LZ
compress
The resource designer has considerable freedom to deviate from these recommendations to accommodate
the presentation requirements of the individual collection.
6
The lower resolution represents a minimum value. Material scanned at less than the minimum value is not
considered to be archival quality. Every effort should be made to scan at the recommended, rather than the
minimum, value.
8 bit
grayscale
file form
PDF or T
with LZ
compress
JFIF, qua
level 50-1